Edema Index

The Edema Index is a black and white map of the retina which can reveal areas of increased fluid accumulation even prior to observing clinically significant macular edema.   The HRT uses a beam of light to scan a 15-degree region of the retina.  Reflected light from the retina returns a characteristic signal.  If the signal is disrupted then the area is highlighted.  Disruption of the reflected light signal is often associated with fluid accumulation.  Characteristic patterns of pathology can be seen in the images, similar to the patterns seen in fluorescein angiography.

However, the information captured by the Edema Index is very different than fluorescein angiography.  Instead of tracking dye in the blood vessels, the software detects the vitreo-retinal interface, then looks for an expected penetration and reflection of light.   When increased interstitial fluid is present, it “stretches” the light signal.  The more the signal is stretched, the higher the Edema Index value.

The Edema Index number is a relative value.   An expected value based on an expected signal return is 1.0.  Values higher than 1.0 indicate disruption of the signal and as the value approaches 1.8 or more, it typically correlates with fluid accumulation.